Anodes



UnitedStates Patent.

ANODES Henry R. Rade, Cleveland, Ohio, assignor to Union Carbide Corporation, a corporation of New :York

No Drawing. Application November 17, 1953 Serial No. 392,745

6 Claims. (Cl. 13 612 6) This invention relates to consumable metal ,anodes'for primary galvanic cells, and to an improved method of constructing such anodes.

Beginning with the early Leclanch type cells, anodes for primary galvanic cells have been cast or fabricated from sheet metal. In relatively large cells such as those used for railway signal service, telephone service, or for flashlights, and similar devices of relatively large size anodes of this type are perfectly satisfactory. Recently, however, due to the development.of electronic devices of smaller size, such as the personal'hearing aid and portable radio transmitters and the like, there has been -a electrolyte. The small anodeof sheet metal tends to'become coated with current-stopping reaction products 'in so short a time: as to renderthe cell commercially-undesirable.

"To counter-balance the decrease inanode' size :necessitated by the continued decrease in cell size, many attempts have been made to increase the effective surface area to-insure morecomplete utilizationofthe anodic material. For example, a sheetmetal anode may be embossed or perforated not only to provide more surface area, but also to prevent adherence to the anode of nonconductive reaction products. While such expedients have been effective -to a degree, they have not solved the problem.

Another way-of effecting-a desired extensionin anode surface areawhich hasbeen adopted to some extent commercially, consists in forming the anode of powdered metal; for example, by compressing powdered anodic material into the form of a cylinder. Anodes so formed have a greater effective surface area than a cylinder of the same size cast from molten metal. However, anodes compressed of metal powders in themselves suffer disadvantages. If an anode so formed is compressed with too little pressure, there results a structurally weak element, while on the other hand if too much pressure is used in the formation of such anodes, an appreciable part of the resultant anode will be unattacked by electrolyte, and therefore, will contribute nothing to the cell reaction. Additionally, powdered metal anodes made in this way are porous, and permit internal electrolytic action during cell discharge with the consequent formation of reaction products within the pores of the anode. Since these reaction products occupy considerably more space than the anode metal, internal pressure develops in the anode, which often results in spalling and breaking apart.

It is the principal object of this invention to provide anode material having greater effective surface area than anodes of: smaller sizefabricated from sheet orcast metal, and yet which is free of the disadvantages'of anodes formed of compressed metal powder, another object of the invention is amethod of producing such anodes.

In accordance withthe invention, these objects are achieved by an anode-composed of finely 'divided anode metal bonded into a'desired shape by an electrolyte-repellent, electrically non-conductive, flexible, thermoplastic material. This invention also includes a method of bonding with a thermoplastic resin finely divided anode metal by compression under heatand pressure.

The most widely used anode material in primary galvanic cells is zinc, andfor the purpose of conciseness, the invention will be describedparticularly with reference to zinc anodes. However, the invention is equally applicable to the production of anodesforme'd of any consumable metal, for example, magnesium or aluminum.

Anodes made in accordance with the invention are composed of finely divided anode metal bonded into the desired shape by a relativelysmall proportion of a thermoplastic hinder, the binder forming a continuous plastic matrix throughout the anode, and the anode metal itself forming a continuous phase. Suitable binding mateirals include polyethylene, polyvinyl chloride, ethyl cellulose, vinyl resins and chlorinated 'diphenyl resins. Of this group, polyethylene ispreferred because'of its excellent heat tolerance.

The proportion of'binder to anode metal'in the anodes of this invention is desirablyas small as possible, commensurate with securing reasonablemechanical strength. Ordinarily, the binder should constitute at least abouti1% by weight of the anode, and 1 largerquantities maybe used, commensurate with the-reduction .in: electrical. conductivity of the formed anodeencountered as the proportion of binder isincreased. For most.purposes,.the proportion of binder need'not exceed 5% ofthe anode, and generally is well below 5%.

The desity ofthe anode, of course, is dependent upon the ratio of metal'toresin in the mixture.

Thus in determining the quantity of thermoplasticbinder to be used in making the anodesof the. invention, ease of molding is an important consideration, but even more important is the necessityfor providing in:the anode=the maximum possiblequantity of active anodemetaL. 'As little as 1% by weight of powdered resin may be used to fill the interstices between the particles, and mixtures containing only this small proportion oftbindermay be molded with relative ease. Whenever greater ease-of molding is desired, largerquantitiesof theresin-may be used, but it must be borne in mind that. the vfinishedoarticle must have satisfactory electrical conductivity for its intended purpose, and the maximum quantity of binder that can be used is that which causes no undue decrease in the electrical conductivity of the finished anode. Numerically, this upper limit is approximately 12%.

In manufacturing the anodes of this invention, powdered anode metal is dry-mixed with powdered binder resin. Although particle size of the metal is not critical, it is convenient to employ powders of such a size that they will pass a 40 mesh (0.417 mm.) screen, while being retained on a mesh (0.088 mm.) screen, and the resin particles should generally be of the same order. The blended mixture is compressed in conventional manner under suitable conditions of elevated temperature and pressure to form any desired shape of anode. Generally, dependent upon the resin selected, the temperature should be at least about 200 F., and the pressure applied, which is in part dependent upon the quality of resin in the mixture, should be at least about 1000 pounds per square inch. Typical examples of suitable mixtures and molding conditions follow.

Cylindrical anodes in outside diameter by height were prepared from a mixture containing 99% zinc having a particle size such as to pass a 40 mesh (0.417 mm.) screen, and 1% polyethylene and successfully molded at 300 F. and a pressure of 2000 pounds per square inch.

From a mixture composed of 97.5% zinc and 2.5% polyethylene, the zinc having a particle size sufficiently small to pass a 40 mesh (0.417 mm.) screen, cup-shaped anodes having an outside diameter of a length of 1 /8" and a side wall thickness of 0.012" were molded at 350 F. and 3000 pounds per square inch pressure.

Anodes in the form of rods of varying lengths having a diameter of were molded at 400 Rand 3500 pounds per square inch pressure, from a blend consisting of 200 grams of powdered zinc and 10 grams of powdered polyethylene, the zinc powder was of a particle size sufliciently small to pass througha 40 mesh (0.417 mm.)

screen.

Anodes typical of those just described when subjected to the action of electrolyte, are uniformly corroded in such a way that complete utilization of the anode metal is achieved. Actually, corrosion, upon which of course battery action depends, proceeds so uniformly and so completely that when the anodes are immersed in electrolyte, complete consumption of the anode metal can be attained, leaving behind a coherent matrix of thermoplastic binder, which still retains the shape of the original anode. Furthermore, reaction proceeds from the outside surfaces of the anode progressively as the anode is consumed without any tendency to breakage ofthe anode by spalling, indicating that electrolytic action within the interior portions of the anode is inhibited.

This inhibition of internal electrolytic action conceivably may be due to a lack of porosity of the anode or to the fact that the binder resin has an electrolyte-repellent property or to both of these phenomena. Whatever the explanation, however, the fact remains that the anodes of the invention are uniformly corroded at the1active surfaces thereof, rather than in the interior portion. This characteristic assures uniform service life of batteries employing the anodes, and prevents early failure of such batteries prior to the complete utilization of the anode metal.

As has been indicated above, a relatively large number of thermoplastic resins may be utilized as the binder in forming the anodes of this invention. Of the group above referred to, polyethylene is preferred because its excellent heat tolerance enables anodesmade in accordance with this invention, utilizing polyethylene asia binder, to be soldered by standard methods without undergoing structural deformation. y

A particular advantage of the invention lies in the fact that anodes of any desired shape may be produced with ease, thereby freeing the battery maker from-the necessity of employing the shapes traditionally used. Further advantages accrue from the fact that pure metal may beemployed, and that uniformity of composition may thereby be achieved. Such uniformity eliminates the possibility of the development of local couples in cells or batteries employing the anodes whichvtend to shorten battery life. Additionally, by reason of the continuity of both matrix and anode metal, substantially complete utilization of the anodic metal may be achieved.

I claim:

1. A primary galvanic cell anode composed of 99% of powdered zinc metal, and 1% of polyethylene powder.

2. An easily soldered primary galvanic cell anode comprising 97 /z% ofzinc. metal and 2 /2% of polyethylene powder. 7 7

3. A cup-shaped primary galvanic cell anode comprising 99% of powdered zinc metal, and 1% of polyethylene powdern;

4. A cup-shaped primary galvanic cell anode comprising 97 /z% of zinc metal, and 2 /2% of polyethylene powder.

5. A molded self-supporting anode for primary galvanic cells, said anode comprising finely divided particles of an anodic metal selected from the group consisting of zinc, aluminum-and magnesium,- said particles being integrally united by a plastic resin selected from the group consisting of polyethylene, ethyl cellulose and chlorinated diphenyl resins, said resin constituting a substantially continuous matrix and being present throughout said anode in an amount ranging from 1- percentto12 percent by weight thereof. e

6. A molded self-supporting anodefor primary galvanic cells, said'anode'comprising finely divided particles of an anodic metal selected fromthe group consisting of zinc, aluminum and magnesium, said particles being integrally united by a plastic resin selected from the group consisting of polyethylene, ethylcellulose and chlorinated diphenyl resins, said resin constituting a substantially continuous matrix, and being present throughout said anode in an amountranging from 1 percent to 5 percent by weight thereof, said metal and said resinr having a particle size ranging from 0.088 millimeter to 0.417 millimeter. 1 e a References Cited in the file of this patent UNITED STATES PATENTS 2,623,915 Moulton- Dec. 30, 1952 2,677,713 Weil et al. May 4, 1954 2,694,743 Ruskin et a1. Nov. 16, 1954 2,708,683 Eisen May 17, 1955 2,737,541 Collidge Mar. 6, 1956 2,738,375 Schlotter Mar. 13, 1956 

1. A PRIMARY GALVANIC CELL ANODE COMPOUND OF 99% OF POWERED ZINC METAL, AND 1% OF POLYETHYLENE POWDER. 